Because SE wants every fight to be cleared by everyone and not have any jobs be shut out just because the fight hates them.

Yes, there is a META, there will always be a META, however, as it stands, it is completely irrelevant. The gap between the 'META' and 'non-META' comps is minimal and not worth worrying over.

And to counter the point that 'everyone can level everything', yes, that is true, however, not everyone wants to level everything. Say you have a META tank pairing of Warrior/Paladin, then the next fight is Dark Knight/Warrior, however, the Paladin tank only wants to level Paladin, they have no desire at all to level any other tank. Should they then be forced to level Warrior against their will just to make the fight easier?

This is just an expanded point against the old cross class skills. If I want to heal and only heal, why should I be forced to level Thaumaturge to learn Swiftcast? If you had no desire to level Paladin but you wanted to level Warrior, should you be forced to level Gladiator to learn Provoke? It is all about giving people the freedom to choose what they want to do, not being forced into it because reasons.